Microwave plasma generator, method of decomposing organic halide, and system for decomposing organic halide

ABSTRACT

A discharge tube ( 11 ) made of a dielectric material extends through a hole ( 3 ) of a rectangular waveguide ( 1 ) and through a coaxial microwave cavity ( 4 ) so as to be coaxial with the central axis of the cavity ( 4 ). This discharge tube ( 11 ) has a double-tube structure including an outer tube ( 12 ) and an inner tube ( 13 ). The sectional area of an annular gap formed between the outer tube ( 12 ) and the inner tube ( 13 ) is held constant over the entire length of the inner tube ( 13 ). This allows the generation of a stable thermal plasma when a reaction gas containing an organic halide and water vapor is supplied into the outer tube through the annular gap with a microwave transmitted from the rectangular waveguide ( 1 ) into the cavity ( 4 ).

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation of Application No. PCT/JP99/05862, filedOct. 22, 1999.

[0002] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 10-302994, filed Oct. 23,1998, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0003] The present invention relates to a microwave plasma generatoruseful to decompose particularly organic halides, such as freon andtrichloromethane, in which the structure of a discharge tube formicrowave plasma generation is improved, and a method and system fordecomposing organic halides such as freon and trichloromethane.

[0004] Large amounts of organic halides such as freon, trichloromethane,and halon, containing fluorine, chlorine, bromine, and the like inmolecules, are used in a variety of applications such as refrigerants,solvents, and fire extinguishers. However, these compounds are highlyvolatile, so many of them are emitted, without being processed, toenvironments such as the air, soil, and water. Such emitted gases arefound to have large influence on environments, e.g., producecarcinogens, destroy the ozonosphere, and cause global warming. Hence,it is necessary to make these organic halides harmless from theviewpoint of environmental protection.

[0005] Conventionally reported organic halide processing methodsprimarily use decomposition reaction at high temperatures. Theseprocessing methods are roughly classified into the incineration methodand the plasma method.

[0006] In the incineration method, volatile organic halides areincinerated together with common wastes such as resins. To burn them ina waste incinerator, corrosion resistance must be improved as a measureagainst hydrogen chloride which is a strong acid and corrosive. Also,since a combustion temperature different from those for combustion ofcommon resins is set, these organic halides must be separately burned.Furthermore, the exhaust amounts of, e.g., hydrogen chloride and dioxineare strictly restricted. As a consequence, the processing amount cannotbe thoughtlessly increased to avoid a primary cause of, e.g., anunstable combustion temperature.

[0007] As the plasma method, a method of decomposing a volatile organichalide into carbon dioxide, hydrogen chloride, and hydrogen fluoride byreacting the halide with water vapor in a plasma is known as a dedicateddecomposing apparatus for, e.g., freon.

[0008] Jpn. Pat. Appln. KOKAI Publication No. 3-222298 has disclosed amicrowave plasma trace element analyzer including a double-structuredischarge tube shown in FIG. 17. A double-structure discharge tube 201shown in FIG. 17 is made of quartz and comprises a cylindrical outertube 202 and a cylindrical inner tube 203. A reaction gas supply pipe204 is connected to the outer tube 202 in the direction of tangent ofthis outer tube 202. A tapered large-diameter cylindrical portion 205 ofthick section is formed at the end portion of the inner tube 203,thereby narrowing the space between the cylindrical portion 205 of thisinner tube 203 and the outer tube 202. The discharge tube 201 isinserted through a microwave cavity 207 having a metal conductor 206 anda rectangular waveguide 208.

[0009] A reaction gas 209 is supplied through the supply pipe 204 to thespace between the outer tube 202 and the inner tube 203 in the dischargetube 201, where the gap between an antenna and a cavity end plate, orbetween inner and outer conductors, in the cavity 207 is positioned.This reaction gas 209 is injected from the exit end of this space. Atthe same time, a carrier gas 210 is supplied into the inner tube 203 andblown off from an injection opening 211 open at the end portion of theinner tube 103, thereby generating a plasma 212 by discharge. Inelemental analysis, for example, nitrogen gas or the like is used as thereaction gas 209, and Ar or He is used as the carrier gas 210.

[0010] The plasma is ignited by using a Tesla coil placed outside andnear the discharge tube 201 at the microwave cavity exit or an ignitioncoil 214 connected to an ignition power supply 213 such as a neontransformer.

[0011] When Ar gas is used as the carrier gas, however, the running costincreases.

[0012] If the flow rate of the carrier gas is increased, the generationof a plasma becomes unstable.

[0013] Also, since the injection opening 211 of the inner tube 203 forinjecting the carrier gas is very small, this portion is readily damagedby, e.g., melting by a plasma.

[0014] Additionally, the gas flow path between the inner tube 203 andthe outer tube 202 is narrowed by the tapered large-diameter cylindricalportion 205 of thick section formed at the end portion of the inner tube203. Although this raises the reaction gas injection rate, the plasma212 flows backward in the space around the cylindrical portion 205.Consequently, the cylindrical portion 205 of the inner tube 203 meltsand breaks.

[0015] On the other hand, to prevent the contact of a plasma with thewall surface of the discharge tube 201, the gas flow in the dischargetube 201 is usually given a swirling flow effect in the gap between theouter tube 202 and the inner tube 203 by connecting the gas supply pipe204 to the outer tube 202 in the direction of tangent of this outer tube202. However, this effect is lost because the thick cylindrical portion205 of the inner tube 203 narrows the flow path formed by the gap.Consequently, even a slight change in the plasma state causes nonuniformdischarge or melts the discharge tube 201.

BRIEF SUMMARY OF THE INVENTION

[0016] It is an object of the present invention to provide a microwaveplasma generator capable of stably and efficiently generating a plasma.

[0017] It is another object of the present invention to provide a methodof decomposing an organic halide capable of efficiently decomposingvolatile organic halides such as freon and trichloromethane.

[0018] It is still another object of the present invention to provide anorganic halide decomposing system capable of efficiently decomposingvolatile organic halides such as freon and trichloromethane.

[0019] A microwave plasma generator according to the present inventioncomprises:

[0020] a rectangular waveguide having a hole to transmit a microwave;

[0021] a coaxial microwave cavity connected to the rectangular waveguideby communicating with the hole; and

[0022] a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends through thecavity so as to be coaxial with a central axis of the cavity,

[0023] wherein the discharge tube has a double-tube structure includingouter and inner tubes,

[0024] a sectional area of an annular gap formed between the outer andinner tubes is constant over an entire length of the inner tube, and

[0025] an end portion of the inner tube has a gas injection openinghaving the same diameter as an internal cavity of the inner tube.

[0026] Another microwave plasma generator according to the presentinvention comprises:

[0027] a rectangular waveguide having a hole to transmit a microwave;

[0028] a cylindrical microwave resonance cavity which is connected tothe rectangular waveguide by communicating with the hole and is placedsuch that a central axis of the resonance cavity aligns with thedirection of electric field in the rectangular waveguide; and

[0029] a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends through theresonance cavity so as to be coaxial with the central axis of thecavity,

[0030] wherein the discharge tube has a double-tube structure includingouter and inner tubes,

[0031] a sectional area of an annular gap formed between the outer andinner tubes is constant over an entire length of the inner tube, and

[0032] an end portion of the inner tube has a gas injection openinghaving the same diameter as an internal cavity of the inner tube.

[0033] In a method of decomposing an organic halide according to thepresent invention, which decomposes an organic halide in a plasma byusing a microwave plasma generator comprising a rectangular waveguidehaving a hole to transmit a microwave, a coaxial microwave cavityconnected to the rectangular waveguide by communicating with the hole,and a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends through thecavity so as to be coaxial with a central axis of the cavity, whereinthe discharge tube has a double-tube structure including outer and innertubes, a sectional area of an annular gap formed between the outer andinner tubes is constant over an entire length of the inner tube, and anend portion of the inner tube has a gas injection opening having thesame diameter as an internal cavity of the inner tube,

[0034] a gas containing the organic halide, water vapor, and air issupplied to the annular gap formed between the outer and inner tubes togenerate a plasma, in the outer tube, which extends from the vicinity ofthe end portion of the inner tube toward an end portion of the outertube.

[0035] In another method of decomposing an organic halide according tothe present invention, which decomposes an organic halide in a plasma byusing a microwave plasma generator comprising a rectangular waveguidehaving a hole to transmit a microwave, a coaxial microwave cavityconnected to the rectangular waveguide by communicating with the hole,and a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends through thecavity so as to be coaxial with a central axis of the cavity, whereinthe discharge tube has a double-tube structure including outer and innertubes, a sectional area of an annular gap formed between the outer andinner tubes is constant over an entire length of the inner tube, and anend portion of the inner tube has a gas injection opening having thesame diameter as an internal cavity of the inner tube,

[0036] the organic halide is supplied into the inner tube, and a gascontaining water vapor and air is supplied to the annular gap formedbetween the outer and inner tubes, thereby generating a plasma, in theouter tube, which extends from the vicinity of the end portion of theinner tube toward an end portion of the outer tube.

[0037] In still another method of decomposing an organic halideaccording to the present invention, which decomposes an organic halidein a plasma by using a microwave plasma generator comprising arectangular waveguide having a hole to transmit a microwave, acylindrical microwave resonance cavity connected to the rectangularwaveguide by communicating with the hole and placed such that a centralaxis of the resonance cavity aligns with the direction of electric fieldin the rectangular waveguide, and a discharge tube which is made of adielectric material, extends through the hole of the rectangularwaveguide, and extends through the resonance cavity so as to be coaxialwith the central axis of the cavity, wherein the discharge tube has adouble-tube structure including outer and inner tubes, a sectional areaof an annular gap formed between the outer and inner tubes is constantover an entire length of the inner tube, and an end portion of the innertube has a gas injection opening having the same diameter as an internalcavity of the inner tube,

[0038] a gas containing the organic halide, water vapor, and air issupplied to the annular gap formed between the outer and inner tubes togenerate a plasma, in the outer tube, which extends from the vicinity ofthe end portion of the inner tube toward an end portion of the outertube.

[0039] In still another method of decomposing an organic halideaccording to the present invention, which decomposes an organic halidein a plasma by using a microwave plasma generator comprising arectangular waveguide having a hole to transmit a microwave, acylindrical microwave resonance cavity connected to the rectangularwaveguide by communicating with the hole and placed such that a centralaxis of the resonance cavity aligns with the direction of electric fieldin the rectangular waveguide, and a discharge tube which is made of adielectric material, extends through the hole of the rectangularwaveguide, and extends through the resonance cavity so as to be coaxialwith the central axis of the cavity, wherein the discharge tube has adouble-tube structure including outer and inner tubes, a sectional areaof an annular gap formed between the outer and inner tubes is constantover an entire length of the inner tube, and an end portion of the innertube has a gas injection opening having the same diameter as an internalcavity of the inner tube,

[0040] the organic halide is supplied into the inner tube, and a gascontaining water vapor and air is supplied to the annular gap formedbetween the outer and inner tubes, thereby generating a plasma, in theouter tube, which extends from the vicinity of the end portion of theinner tube toward an end portion of the outer tube.

[0041] Another microwave plasma generator according to the presentinvention comprises:

[0042] a rectangular waveguide having a hole to transmit a microwave;

[0043] a cylindrical microwave resonance cavity which is connected tothe rectangular waveguide by communicating with the hole and is placedsuch that a central axis of the resonance cavity aligns with thedirection of electric field in the rectangular waveguide;

[0044] a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends through theresonance cavity so as to be coaxial with the central axis of thecavity;

[0045] a metal conductor which is connected, while being fitted in thedischarge tube, to a portion of the discharge tube near the hole, andextends into the resonance cavity through the hole; and

[0046] a slidable probe antenna which is interposed between the metalconductor and the discharge tube so as to be slidable in an axialdirection of the discharge tube, and extends into the resonance cavitythrough the hole in the rectangular waveguide.

[0047] Still another microwave plasma generator according to the presentinvention comprises:

[0048] a rectangular waveguide having a hole to transmit a microwave;

[0049] a cylindrical microwave resonance cavity which is connected tothe rectangular waveguide by communicating with the hole, is placed suchthat a central axis of the resonance cavity aligns with the direction ofelectric field in the rectangular waveguide, and has an end plate on abottom portion;

[0050] a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends through theresonance cavity so as to be coaxial with the central axis of thecavity;

[0051] a metal conductor which is connected, while being fitted in thedischarge tube, to a portion of the discharge tube near the hole, andextends into the resonance cavity through the hole; and

[0052] an annular metal conductor which is interposed between the endplate of the resonance cavity and the discharge tube extending throughthe end plate, and extends from the outside to the inside of the cavity.

[0053] Still another microwave plasma generator according to the presentinvention comprises:

[0054] a rectangular waveguide having a hole to transmit a microwave;

[0055] a cylindrical microwave resonance cavity which is connected tothe rectangular waveguide by communicating with the hole, is placed suchthat a central axis of the resonance cavity aligns with the direction ofelectric field in the rectangular waveguide, and has an end plate on abottom portion;

[0056] a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends through theresonance cavity so as to be coaxial with the central axis of thecavity; and

[0057] a metal conductor which is connected, while being fitted in thedischarge tube, to a portion of the discharge tube near the hole, andextends into the resonance cavity through the hole,

[0058] wherein the end plate of the resonance cavity has a taperedprojection, which projects toward the metal conductor, in a portionthrough which the discharge tube extends.

[0059] In a method of decomposing an organic halide according to thepresent invention, a thermal plasma is generated by irradiating a gascontaining an organic halide with a microwave, thereby decomposing theorganic halide.

[0060] A system for decomposing an organic halide according to thepresent invention comprises:

[0061] a rectangular waveguide having a hole to transmit a microwave;

[0062] a coaxial microwave cavity connected to the rectangular waveguideby communicating with the hole;

[0063] a reaction tube placed below the cavity;

[0064] a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends into thereaction tube through the cavity so as to be coaxial with the centralaxis of the cavity;

[0065] a metal conductor which is connected, while being fitted in thedischarge tube, to a portion of the discharge tube near the hole, andextends into the resonance cavity through the hole;

[0066] a vessel into which a lower end of the reaction tube is insertedand which contains an aqueous alkali solution; and

[0067] gas supply means for supplying a gas containing an organic halideto the discharge tube.

[0068] Another system for decomposing an organic halide according to thepresent invention comprises:

[0069] a rectangular waveguide having a hole to transmit a microwave;

[0070] a coaxial microwave cavity connected to the rectangular waveguideby communicating with the hole;

[0071] a reaction tube placed below the cavity;

[0072] a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends into thereaction tube through the cavity so as to be coaxial with the centralaxis of the cavity;

[0073] a metal conductor which is connected, while being fitted in thedischarge tube, to a portion of the discharge tube near the hole, andextends into the resonance cavity through the hole;

[0074] a vessel into which a lower end of the reaction tube is insertedand which contains an aqueous alkali solution;

[0075] gas supply means for supplying a gas containing an organic halideto the discharge tube through first piping;

[0076] water supply means connected to the first piping through secondpiping; and

[0077] heating means, provided for the second piping, for convertingwater flowing in the second piping into water vapor.

[0078] Still another system for decomposing an organic halide accordingto the present invention comprises:

[0079] a rectangular waveguide having a hole to transmit a microwave;

[0080] a coaxial microwave cavity connected to the rectangular waveguideby communicating with the hole;

[0081] a reaction tube placed below the cavity;

[0082] a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends into thereaction tube through the cavity so as to be coaxial with the centralaxis of the cavity;

[0083] a metal conductor which is connected, while being fitted in thedischarge tube, to a portion of the discharge tube near the hole, andextends into the resonance cavity through the hole;

[0084] a vessel into which a lower end of the reaction tube is insertedand which contains an aqueous alkali solution;

[0085] gas supply means for supplying a gas containing an organic halideto the discharge tube; and

[0086] water vapor spraying means, formed in a circumferential wall ofthe reaction tube, for spraying water vapor into a thermal plasmageneration region near a lower end portion of the discharge tube.

[0087] Still another system for decomposing an organic halide accordingto the present invention comprises:

[0088] a rectangular waveguide having a hole to transmit a microwave;

[0089] a coaxial microwave cavity connected to the rectangular waveguideby communicating with the hole;

[0090] a reaction tube placed below the cavity;

[0091] a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends into thereaction tube through the cavity so as to be coaxial with the centralaxis of the cavity;

[0092] a metal conductor which is connected, while being fitted in thedischarge tube, to a portion of the discharge tube near the hole, andextends into the resonance cavity through the hole;

[0093] a vessel into which a lower end of the reaction tube is insertedand which contains an aqueous alkali solution;

[0094] gas supply means for supplying a gas containing an organic halideto the discharge tube; and

[0095] alkali water spraying means, formed in a side wall of thereaction tube, for spraying alkali water into a thermal plasmageneration region near a lower end portion of the discharge tube.

[0096] Still another system for decomposing an organic halide accordingto the present invention comprises:

[0097] a rectangular waveguide having a hole to transmit a microwave;

[0098] a coaxial microwave cavity connected to the rectangular waveguideby communicating with the hole;

[0099] a reaction tube placed below the cavity;

[0100] a discharge tube which is made of a dielectric material, extendsthrough the hole of the rectangular waveguide, and extends into thereaction tube through the cavity so as to be coaxial with the centralaxis of the cavity;

[0101] a metal conductor which is connected, while being fitted in thedischarge tube, to a portion of the discharge tube near the hole, andextends into the resonance cavity through the hole;

[0102] a vessel into which a lower end of the reaction tube is insertedand which contains an aqueous alkali solution;

[0103] gas supply means for supplying a gas containing an organic halideto the discharge tube; and

[0104] heating means, installed in at least one of a position around thereaction tube and a position below the vessel, for vaporizing theaqueous alkali solution in the vessel and introducing water vapor into athermal plasma generation region near a lower end portion of thedischarge tube.

[0105] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0106] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiments of the invention, and together with the general descriptiongiven above and the detailed description of the preferred embodimentsgiven below, serve to explain the principles of the invention.

[0107]FIG. 1 is a sectional view showing an organic halide decomposingsystem including a microwave plasma generator according to the firstembodiment of the present invention;

[0108]FIG. 2 is a sectional view of the microwave plasma generator shownin FIG. 1;

[0109]FIG. 3 is a sectional view showing an organic halide decomposingsystem including a microwave plasma generator according to the secondembodiment of the present invention;

[0110]FIG. 4 is a sectional view of the microwave plasma generator shownin FIG. 3;

[0111]FIG. 5 is a sectional view showing a microwave plasma generatoraccording to the third embodiment of the present invention;

[0112]FIG. 6 is a sectional view showing a microwave plasma generatoraccording to the fourth embodiment of the present invention;

[0113]FIG. 7 is a sectional view showing an organic halide decomposingsystem including a microwave plasma generator according to the fifthembodiment of the present invention;

[0114]FIG. 8 is a sectional view of the microwave plasma generator shownin FIG. 7;

[0115]FIG. 9 is a sectional view showing a microwave plasma generatoraccording to the sixth embodiment of the present invention;

[0116]FIG. 10 is a sectional view showing a microwave plasma generatoraccording to the seventh embodiment of the present invention;

[0117]FIG. 11 is a sectional view showing a microwave plasma generatoraccording to the eighth embodiment of the present invention;

[0118]FIG. 12 is a sectional view showing an organic halide decomposingsystem according to the ninth embodiment of the present invention;

[0119]FIG. 13 is a sectional view showing an organic halide decomposingsystem according to the 10th embodiment of the present invention;

[0120]FIG. 14 is a sectional view showing an organic halide decomposingsystem according to the 11th embodiment of the present invention;

[0121]FIG. 15 is a sectional view showing an organic halide decomposingsystem according to the 12th embodiment of the present invention;

[0122]FIG. 16 is a sectional view showing an organic halide decomposingsystem according to the 13th embodiment of the present invention; and

[0123]FIG. 17 is a sectional view showing a conventional microwaveplasma generator.

DETAILED DESCRIPTION OF THE INVENTION

[0124] Microwave plasma generators and methods and systems fordecomposing an organic halide (e.g., freon gas) by using a thermalplasma according to embodiments of the present invention will bedescribed below with reference to the accompanying drawings.

[0125] (First Embodiment)

[0126]FIG. 1 is a sectional view showing an organic halide (e.g., freon)decomposing system including a microwave plasma generator having acoaxial microwave cavity according to this first embodiment. FIG. 2 is asectional view of the microwave plasma generator shown in FIG. 1.

[0127] Referring to FIG. 1, a horizontally extending rectangularwaveguide 1 has a microwave oscillator 2, which oscillates a microwavewith a frequency of 2.45 GHz, in its start end portion (left-hand end),and has a hole 3 in this end portion. This rectangular waveguide 1transmits a microwave from the start end portion toward a terminal endportion (right-hand end).

[0128] As shown in FIG. 2, a coaxial microwave cavity 4 is so connectedas to extend vertically by communicating with the hole 3 of therectangular waveguide 1. This cavity 4 includes an end plate 5, acylindrical outer conductor 6, a metal conductor 8, and a cylindricalinner conductor 9. The end plate 5 is positioned on the bottom portionof the cavity 4, and the outer conductor 6 is fixed on this end plate 5.The metal conductor 8 is fixed to the rectangular waveguide 1 near thehole 3 and has an inverse conical portion 7 extended into the outerconductor 6. A hole is formed through the center of this metal conductor8. The inner conductor 9 extends from the lower end of the inverseconical portion 7 of the metal conductor 8 toward a position below themiddle position of the outer conductor 6 and is placed coaxially withthe outer conductor 8. Reference numeral 4 a denotes a throttle platewhich is formed on the upper surface of the cavity 4 and defines thehole 3 of the rectangular waveguide 1, i.e., the connecting portionbetween the rectangular waveguide 1 and the cavity 4. A reaction tube 10is placed below the coaxial microwave cavity 4.

[0129] A discharge tube 11 made of a dielectric material, e.g., quartz,extends through the end plate 5 from the hole in the metal conductor 8via the inner conductor 9 and is inserted into the reaction tube 10.That is, this discharge tube 11 is so placed as to be coaxial with thecentral axis of the coaxial microwave cavity 4 and extends through therectangular waveguide 1 and the cavity 4.

[0130] The discharge tube 11 has a double-tube structure including anouter tube 12 whose upper end is closed and an inner tube 13 coaxiallyinserted into this outer tube 12 from its upper end. Both the outer andinner tubes 12 and 13 are straight tubes. That is, in its end portion(lower end portion) the inner tube 13 has a gas injection opening havingthe same diameter as the internal cavity of this inner tube 13. Thesectional area of an annular gap formed between the outer and innertubes 12 and 13, i.e., the sectional area of the annular gap formedbetween the outer and inner tubes 12 and 13 in a section perpendicularto the longitudinal direction of the discharge tube 11, is constant overthe entire length of the inner tube 13.

[0131] Freon from a freon gas container 14, air from an air container15, argon from an argon container 16, and water vapor from a water vaporgenerator 17 are supplied through a gas supply pipe 18 to a portionabove the annular gap formed between the outer and inner tubes 12 and 13of the discharge tube 11. The gas supply pipe 18 is connected along thedirection of tangent of the outer tube 12.

[0132] As shown in FIG. 2, an ignition electrode 20 connected to anignition power supply 19 outside the outer tube 12 of the discharge tube11 ignites a thermal plasma.

[0133] The lower end of the reaction tube 10 is dipped into an aqueousalkali solution 22 in a vessel 21. An exhaust duct 23 is connected tothe upper portion of this vessel 21.

[0134] The function of the aforementioned microwave plasma generator anda method of decomposing an organic halide, e.g., freon, by using theorganic halide decomposing system including this generator will bedescribed below.

[0135] The microwave oscillator 2 is operated to generate a microwave.This microwave is transmitted by the rectangular waveguide 1 and thentransmitted to the coaxial microwave cavity 4 through the metalconductor 8 and the inner conductor 9. As a consequence, in the cavity 4an axial-direction electric field is formed between the inner conductor9 and the end plate 5.

[0136] With the microwave thus transmitted into the coaxial microwavecavity 4, freon gas and a gas containing water vapor are suppliedthrough the gas supply pipe 18 into the annular gap formed between theouter and inner tubes 12 and 13 of the discharge tube 11, and theignition power supply 19 is operated to allow the ignition electrode 20to discharge. Consequently, a thermal plasma 24 having high electronenergy and a temperature of 2,000 to 6,000 K is generated in thedischarge tube 11. This thermal plasma 24 extends from the dischargetube 11 into the reaction tube 10 at the lower end of the discharge tube11. This makes the freon gas readily dissociable into chlorine,fluorine, and hydrogen atoms, so the freon gas reacts with water vapor.For example, freon R12 (CCl₂F₂) used in a refrigerant of anair-conditioner is easily decomposed by reaction indicated by

CCl₂F₂+2H₂O→2HCl+2HF+CO₂  (1)

[0137] Also, even freon 134a (CF₃CH₂F) known as alternate freon notcontaining chlorine and assumed to be comparatively difficult todecompose is readily decomposed by reaction indicated by

CF₃CH₂F+2H₂O→4HF+CO₂+C  (2)

[0138] C converts to CO₂ in the presence of oxygen or the like.

[0139] The decomposed gas from the reaction tube 10 is made harmless byreaction indicated by formula 3 below by passing through the aqueousalkali solution (e.g., calcium hydroxide) 22 contained in the vessel 21.The gas containing carbonic acid gas and the like is exhausted from theexhaust duct 23.

2HCl+2HF+2Ca(OH)₂→CaCl₂+CaF₂+4H₂O  (3)

[0140] In the microwave plasma generator according to the firstembodiment as described above, a straight tube is used as the inner tube13 forming the double-tube structure of the discharge tube 11 so thatthe sectional area of the annular gap formed between this inner tube 13and the outer tube 12 is constant. Accordingly, the gas described abovecan be injected at high flow rate from the annular gap, and thisenhances the gas blowing effect. As a consequence, a stable thermalplasma can be generated. Therefore, once this thermal plasma is ignited,disappearance of the thermal plasma can be prevented even when thesupply of Ar gas as a carrier gas is stopped to switch to freon or watervapor. This can suppress the consumption of Ar gas.

[0141] Also, the injection opening of the inner tube 13 is made largerthan the conventional inner tube injection opening shown in FIG. 17.This can prevent damage to the inner tube 13 by the thermal plasma 24and also prevent adhesion of soot and condensation of water vapor.

[0142] Furthermore, the gas supply pipe 18 is connected to the dischargetube 11 along the direction of tangent of the outer tube 12. Therefore,when the gas is supplied to the aforementioned annular gap through thegas supply pipe 18, the gas can be swirled as indicated by the arrow inFIG. 2. Since the sectional area of the annular gap formed between theinner and outer tubes 13 and 12 is held constant, this swirling floweffect reaches the exit of the annular gap, and the gas is injected intothe plasma generation region. Hence, the flow rate on the inner wall ofthe outer tube 12 can be raised as when the gap between the inner andouter tubes is narrowed by increasing the diameter of the exit of theinner tube in the conventional microwave plasma generator shown in FIG.17. Also, the flow amount of gas injected from the gap can be increasedas described previously. Consequently, in a portion of the outer tube 12near the exit of the gap, the effect of blowing off the gas from theinner wall of the outer tube 12 increases. This generates a stablerthermal plasma near the center of the outer tube 12. Accordingly,disappearance of the thermal plasma can be prevented even when thesupply of Ar gas as a carrier gas is stopped to switch to freon or watervapor. This can suppress the consumption of Ar gas.

[0143] Moreover, an organic halide such as freon can be easilydecomposed and made harmless by the use of the decomposing system asshown in FIG. 1.

[0144] In the first embodiment described above, the ignition electrodeis placed near the portion of the discharge tube extending outside thecavity. However, the ignition electrode can also be inserted into theinner tube of the discharge tube. Alternatively, an ignition electrodein the form of a coil can be inserted into the inner tube of thedischarge tube.

[0145] Also, in the first embodiment described above, freon gas and agas containing water vapor are supplied into the annular gap formedbetween the outer and inner tubes 12 and 13 of the discharge tube 11,thereby decomposing the freon. However, the present invention is notlimited to this embodiment. For example, it is also possible to supplythe organic halide into the inner tube 13 and supply water vapor to theannular gap formed between the outer and inner tubes 12 and 13 togenerate in the outer tube 12 a plasma which extends from the endportion of the inner tube 13 to the end portion of the outer tube 12,thereby decomposing the freon.

EXAMPLE 1

[0146] In this Example 1, the organic halide decomposing systemincluding the microwave plasma generator according to the firstembodiment described above was used to decompose freon R12 (CCl₂F₂) andfreon 134a (CH₂CF₄) under the conditions explained below.

[0147] That is, the inside diameter and length of the coaxial microwavecavity 4 for generating a thermal plasma shown in FIGS. 1 and 2 were setto 40 mm and 50 mm, respectively, and the gap length between the innerconductor 9 and the end plate 5 of the cavity 4 was set to 20 mm.

[0148] Inside the cavity 4, the quartz discharge tube 11 ran through themetal conductor 8, the inner conductor 9, and the end plate 5. Thisdischarge tube 11 was composed of the outer tube 12 (outside diameter 13mm, inside diameter 10 mm) and the inner tube 13 (outside diameter 6 mm,inside diameter 4 mm).

[0149] To the discharge tube 11 in the cavity 4, freon (R12) wassupplied at the atmospheric pressure and a flow rate of 6 L/min from thefreon container 14, and water vapor was supplied at the atmosphericpressure and a flow rate of 12 L/min from the water vapor generator 17,both through the gas supply pipe 18. A 2.45-GHz microwave was introducedfrom the oscillator 2 into the coaxial microwave cavity 4 via the metalconductor 8 mounted on the rectangular waveguide 1 and the innerconductor 9, thereby discharging by an axial-direction electric fieldformed between the inner conductor 9 and the end plate 5. This dischargewas found to be well stable even at the atmospheric pressure compared todischarge using a conventional apparatus.

[0150] The decomposition ratio of freon was measured by sampling aportion of the gas from the reaction tube 10 and calculating thepresence/absence of a thermal plasma from gas chromatographic analysisof the freon concentration. Freon and water vapor were supplied to theinner tube and the outer tube (the gap between the inner and outertubes) of the discharge tube by different methods. Table 1 below showsthe decomposition ratio measurement results obtained by gaschromatographic analysis when the freon supply amount was 1 kg/h and themicrowave power was 1,200 W.

[0151] The same test was conducted on freon 134a into which air wasmixed. The results are also shown in Table 1. TABLE 1 Freon Water vaporOuter tube Outer tube Quality of Decomposition Decomposition flow rateInner tube flow rate Inner tube discharge* ratio (R12) ratio (134a) 6L/min 0 L/min  0 L/min 12 L/min  D — — 3 L/min 3 L/min  6 L/min 6 L/minD — — 0 L/min 6 L/min 12 L/min 0 L/min D — — 6 L/min 0 L/min 12 L/min 0L/min A 99.99 or more 99.99% or more

[0152] As is apparent from Table 1, when freon and water vapor weresupplied only to the annular gap between the outer and inner tubes 12and 13 of the discharge tube 11, a thermal plasma was ignited, and asatisfactory decomposition ratio was obtained. Similar decomposition wasalso possible for freon 134a.

[0153] Note that it was experimentally confirmed that even when argon,air, or the like was mixed as an additional gas, a substantially equaldecomposition ratio was obtained by controlling the microwave power.

[0154] (Second Embodiment)

[0155]FIG. 3 is a sectional view showing an organic halide (e.g., freon)decomposing system including a microwave plasma generator having acylindrical microwave resonance cavity according to the secondembodiment of the present invention. FIG. 4 is a sectional view of themicrowave plasma generator shown in FIG. 3. The same reference numeralsas in FIGS. 1 and 2 denote the same parts in FIGS. 3 and 4, and adetailed description thereof will be omitted.

[0156] Referring to FIG. 3, a horizontally extending rectangularwaveguide 1 has a microwave oscillator 2, which oscillates a microwavewith a frequency of 2.45 GHz, in its start end portion (left-hand end),and has a hole 3 in this end portion. This rectangular waveguide 1transmits a microwave from the start end portion toward a terminal endportion (right-hand end).

[0157] As shown in FIG. 4, a cylindrical microwave resonance cavity 25is so connected as to extend vertically by communicating with the hole 3of the rectangular waveguide 1. This cavity 25 includes an end plate 26,a cylindrical outer conductor 27, a metal conductor 29, and acylindrical probe antenna 30. The end plate 26 is positioned on thebottom portion of the cavity 25 and has a larger diameter than that ofthe coaxial microwave cavity shown in FIG. 2. The outer conductor 27 isfixed on this end plate 26. The metal conductor 29 is fixed to therectangular waveguide 1 near the hole 3 and has an inverse conicalportion 28 extended into the outer conductor 27. A hole is formedthrough the center of this metal conductor 29. The probe antenna 30extends from the lower end of the inverse conical portion 28 of themetal conductor 29 into the outer conductor 27 and is placed coaxiallywith the outer conductor 27. Reference numeral 25 a denotes a throttleplate which is formed on the upper surface of the cavity 25 and definesthe hole 3 of the rectangular waveguide 1, i.e., the connecting portionbetween the rectangular waveguide 1 and the cavity 25.

[0158] A discharge tube 11 made of a dielectric material, e.g., quartz,runs through the end plate 26 from the hole in the metal conductor 29via the probe antenna 30 and is inserted into a reaction tube 10. Thatis, this discharge tube 11 is so placed as to be coaxial with thecentral axis of the cylindrical microwave resonance cavity 25 andextends through the rectangular waveguide 1 and the cavity 25.

[0159] The discharge tube 11 has a double-tube structure including anouter tube 12 whose upper end is closed and an inner tube 13 coaxiallyinserted into this outer tube 12 from its upper end. Both the outer andinner tubes 12 and 13 are straight tubes. That is, in its end portion(lower end portion) the inner tube 13 has a gas injection opening havingthe same diameter as the internal cavity of this inner tube 13. Thesectional area of an annular gap formed between the outer and innertubes 12 and 13, i.e., the sectional area of the annular gap formedbetween the outer and inner tubes 12 and 13 in a section perpendicularto the longitudinal direction of the discharge tube 11, is constant overthe entire length of the inner tube 13.

[0160] The function of the aforementioned microwave plasma generator anda method of decomposing an organic halide, e.g., freon, by using theorganic halide decomposing system including this generator will bedescribed below.

[0161] The microwave oscillator 2 is operated to generate a microwave.This microwave is transmitted by the rectangular waveguide 1 and thentransmitted to the cylindrical microwave resonance cavity 25 through themetal conductor 29 and the probe antenna 30. As a consequence, in thecavity 25 a large axial-direction electric field of TM₀₁₀ mode formsbetween the probe antenna 30 and the end plate 26. Additionally, thiselectric field in the cavity 25 is stable because the metal conductor 29and the probe antenna 30 couple the electric field mode in therectangular waveguide 1 and the electric field mode in the cylindricalmicrowave resonance cavity 25.

[0162] With the microwave thus transmitted into the cylindricalmicrowave resonance cavity 25, freon gas and a gas containing watervapor are supplied through a gas supply pipe 18 into the annular gapformed between the outer and inner tubes 12 and 13 of the discharge tube11, and an ignition power supply 19 is operated to allow an ignitionelectrode 20 to discharge. Consequently, a thermal plasma 24 having highelectron energy and a temperature of 2,000 to 6,000 K is generated inthe discharge tube 11. This thermal plasma 24 extends from the dischargetube 11 into the reaction tube 10 at the lower end of the discharge tube11. This makes the freon gas readily dissociable into chlorine,fluorine, and hydrogen atoms, so the freon gas reacts with water vapor.For example, freon R12 (CCl₂F₂) is easily decomposed by reactionindicated by formula 1 presented earlier.

[0163] The decomposed gas from the reaction tube 10 is made harmless byreaction indicated by formula 3 presented earlier by passing through anaqueous alkali solution (e.g., calcium hydroxide) 22 contained in avessel 21. The gas containing carbonic acid gas and the like isexhausted from an exhaust duct 23.

[0164] In the microwave plasma generator according to the secondembodiment as described above, a straight tube is used as the inner tube13 forming the double-tube structure of the discharge tube 11 so thatthe sectional area of the annular gap formed between this inner tube 13and the outer tube 12 is constant. Accordingly, the gas described abovecan be injected at high flow rate from the annular gap, and thisenhances the gas blowing effect. As a consequence, a stable thermalplasma can be generated. Therefore, once this thermal plasma is ignited,disappearance of the thermal plasma can be prevented even when thesupply of Ar gas as a carrier gas is stopped to switch to freon or watervapor. This can suppress the consumption of Ar gas.

[0165] Also, the injection opening of the inner tube 13 is made largerthan the conventional inner tube injection opening shown in FIG. 17.This can prevent damage to the inner tube 13 by the thermal plasma 24and also prevent adhesion of soot and condensation of water vapor.

[0166] Furthermore, the gas supply pipe 18 is connected to the dischargetube 11 along the direction of tangent of the outer tube 12. Therefore,when the gas is supplied to the aforementioned annular gap through thegas supply pipe 18, the gas can be swirled as indicated by the arrow inFIG. 4. Since the sectional area of the annular gap formed between theinner and outer tubes 13 and 12 is held constant, this swirling floweffect reaches the exit of the annular gap, and the gas is injected intothe plasma generation region. Hence, the flow rate on the inner wall ofthe outer tube 12 can be raised as when the gap between the inner andouter tubes is narrowed by increasing the diameter of the exit of theinner tube in the conventional microwave plasma generator shown in FIG.17. Also, the flow amount of gas injected from the gap can be increasedas described previously. Consequently, in a portion of the outer tube 12near the exit of the gap, the effect of blowing off the gas from theinner wall of the outer tube 12 increases. This generates a stablerthermal plasma near the center of the outer tube 12. Accordingly,disappearance of the thermal plasma can be prevented even when thesupply of Ar gas as a carrier gas is stopped to switch to freon or watervapor. This can suppress the consumption of Ar gas.

[0167] Moreover, an organic halide such as freon can be easilydecomposed and made harmless by the use of the decomposing system asshown in FIG. 3.

EXAMPLE 2

[0168] In this Example 2, the organic halide decomposing systemincluding the microwave plasma generator according to the secondembodiment described above was used to decompose freon R12 under theconditions explained below.

[0169] That is, the inside diameter and length of the cylindricalmicrowave resonance cavity 25 for generating a thermal plasma shown inFIGS. 3 and 4 were set to 90 mm and 50 mm, respectively, and the gaplength between the probe antenna 30 and the end plate 26 of thecylindrical microwave resonance cavity 25 was set to 20 mm.

[0170] The discharge tube 11 was composed of the outer tube 12 (outsidediameter 13 mm, inside diameter 10 mm) and the inner tube 13 (outsidediameter 6 mm, inside diameter 4 mm).

[0171] To the discharge tube 11 in the cavity 25, freon (R12) wassupplied at the atmospheric pressure and a flow rate of 6 L/min from afreon container 14, and water vapor was supplied at the atmosphericpressure and a flow rate of 12 L/min from a water vapor generator 17,both through the gas supply pipe 18. A 2.45-GHz microwave was introducedfrom the oscillator 2 into the cylindrical microwave resonance cavity 25via the metal conductor 29 mounted on the rectangular waveguide 1 andthe probe antenna 30, thereby discharging by an axial-direction electricfield of TM₀₁₀ mode formed between the probe antenna 30 and the endplate 26.

[0172] By setting the freon supply amount to 1 kg/h and setting thewater vapor supply amount such that the water/freon molar ratio was 2,freon and water vapor were supplied to the inner tube and the outer tube(the gap between the inner and outer tubes) of the discharge tube bydifferent methods. In this manner the decomposition ratio was measured.The results are shown in Table 2 below. TABLE 2 Freon Water vapor OuterOuter tube tube flow Inner flow Inner Quality of Decomposition rate tuberate tube discharge* ratio 6 L/min 0 L/min  0 L/min 12 L/min  D — 3L/min 3 L/min  6 L/min 6 L/min D — 0 L/min 6 L/min 12 L/min 0 L/min D —6 L/min 0 L/min 12 L/min 0 L/min A 99.99 or more 6 L/min 0 L/min 10L/min 2 L/min B 99% 4 L/min 2 L/min 12 L/min 0 L/min B 99%

[0173] As is apparent from Table 2, when freon and water vapor weresupplied only to the annular gap between the outer and inner tubes 12and 13 of the discharge tube 11, a thermal plasma was generated moststably, and the freon was decomposed at high decomposition ratio.

[0174] On the other hand, even when freon or water vapor was supplied tothe inner tube 13, a thermal plasma was generated if the supply amountwas small.

[0175] Table 3 below shows the microwave power dependence of the freondecomposition ratio when the freon flow rate and the water vapor flowrate were set to 6 L/min and 12 L/min, respectively, in the structureshown in FIGS. 3 and 4 in which the gas supply pipe 18 was connected tothe outer tube 12 of the discharge tube 11 along the direction oftangent of this outer tube 12. Table 3 also shows the comparison of thepresence/absence of melting of the discharge tube when the conventionaldischarge tube shown in FIG. 17 was used. TABLE 3 Melting of Microwavedischarge Decomposition power tube ratio Prior art  500 W Not   80% Notmelted melted Not   90% Softened 1000 W melted slightly 1500 W Not99.99% Softened melted largely 2000 W Not 99.99% or Melted melted more

[0176] As shown in Table 3, the discharge tube of the conventionalapparatus melted as the microwave power increased. In the presentinvention, however, melting of the discharge tube could be suppressedeven when the microwave power was raised.

[0177] (Third Embodiment)

[0178]FIG. 5 is a sectional view showing another form of the microwaveplasma generator having the cylindrical microwave resonance cavityincorporated into the organic halide decomposing system according to thesecond embodiment described above. The same reference numerals as inFIGS. 3 and 4 denote the same parts in FIG. 5, and a detaileddescription thereof will be omitted.

[0179] As shown in FIG. 5, this microwave plasma generator has astructure in which an ignition electrode 20 connected to an ignitionpower supply 19 is inserted into an inner tube 13 of a double-tubedischarge tube 11.

[0180] Similar to the aforementioned second embodiment, the microwaveplasma generator having this construction can reduce the consumptionamount of argon gas during the generation of a thermal plasma andsuppress damage to the inner tube by the thermal plasma.

[0181] Additionally, since the ignition electrode 20 is inserted intothe inner tube 13 of the discharge tube 11, ignition can be stablyperformed with high reproducibility regardless of the thermal plasmastate. Consequently, a thermal plasma can be ignited even withlow-flow-rate Ar gas.

[0182] Freon gas and a gas containing water vapor are supplied through agas supply pipe 18 into an annular gap formed between an outer tube 12and the inner tube 13 of the discharge tube 11 of an organic halidedecomposing system including the microwave plasma generator shown inFIG. 5, and the ignition power supply 19 is operated to allow theignition electrode 20 inserted into the inner tube 13 to discharge. As aconsequence, as in the second embodiment described above, freon such asfreon R12 can be readily decomposed by reaction indicated by formula 1presented earlier.

EXAMPLE 3

[0183] In this Example 3, the organic halide decomposing systemincluding the microwave plasma generator according to the thirdembodiment described above was used to decompose freon gas under theconditions explained below.

[0184] That is, a high-frequency voltage generated by the power supply19 composed of a Tesla coil shown in FIG. 5 was introduced to theignition electrode 20 inserted into the inner tube 13 of the dischargetube 11, and Ar gas was supplied to the annular gap between the outerand inner tubes 12 and 13 to discharge. After a thermal plasma wasignited, this thermal plasma was not lost even when the gas supplied tothe annular gap was switched to freon and water vapor. Table 4 belowshows the results measured by changing the Ar flow rate. Table 4 alsoshows the results of the prior art using the microwave plasma generatorshown in FIG. 17. TABLE 4 Quality of discharge* Ar flow rate Presentinvention Prior art  2 L/min B D  5 L/min A C 10 L/min A B

[0185] As is evident from Table 4 above, the microwave plasma generatorof Example 3 reliably ignited a thermal plasma even at low flow rates atwhich ignition was difficult to perform by the conventional apparatus.

[0186] (Fourth Embodiment)

[0187]FIG. 6 is a sectional view showing another form of the microwaveplasma generator having the cylindrical microwave resonance cavityincorporated into the organic halide decomposing system according to thesecond embodiment described above. The same reference numerals as inFIGS. 3 and 4 denote the same parts in FIG. 6, and a detaileddescription thereof will be omitted.

[0188] As shown in FIG. 6, this microwave plasma generator has astructure in which a coiled ignition electrode 31 connected to anignition power supply 19 is inserted into an inner tube 13 of adouble-tube discharge tube 11.

[0189] Similar to the aforementioned second embodiment, the microwaveplasma generator having this construction can reduce the consumptionamount of argon gas during the generation of a thermal plasma andsuppress damage to the inner tube by the thermal plasma.

[0190] Additionally, since the coiled ignition electrode 31 is insertedinto the inner tube 13 of the discharge tube 11, ignition can be stablyperformed with high reproducibility regardless of the thermal plasmastate. Consequently, a thermal plasma can be ignited even withlow-flow-rate Ar gas.

[0191] Freon gas and a gas containing water vapor are supplied through agas supply pipe 18 into an annular gap formed between an outer tube 12and the inner tube 13 of the discharge tube 11 of an organic halidedecomposing system including the microwave plasma generator shown inFIG. 6, and the ignition power supply 19 is operated to allow the coiledignition electrode 31 inserted into the inner tube 13 to discharge. As aconsequence, as in the second embodiment described above, freon such asfreon R12 can be readily decomposed by reacting as indicated by formula1 presented earlier.

EXAMPLE 4

[0192] In this Example 4, the organic halide decomposing systemincluding the microwave plasma generator according to the fourthembodiment described above was used to decompose freon gas under theconditions explained below.

[0193] That is, a high-frequency voltage generated by the power supply19 composed of a Tesla coil shown in FIG. 6 was introduced to the coiledignition electrode 31 inserted into in the inner tube 13 of thedischarge tube 11, and Ar gas was supplied to the annular gap betweenthe outer and inner tubes 12 and 13 to discharge. After a thermal plasmawas ignited, this thermal plasma was not lost even when the gas suppliedto the annular gap was switched to freon and water vapor. Table 5 belowshows the results measured by changing the Ar flow rate and the resultsmeasured when Ar mixed with moisture was supplied. Table 5 also showsthe results of the prior art using the microwave plasma generator shownin FIG. 17. TABLE 5 Quality of discharge* Ar flow rate Present inventionPrior art  2 L/min A D  5 L/min A C 10 L/min A B 10 L/min A C (Moisturepresent)

[0194] As is apparent from Table 5 above, the microwave plasma generatorof Example 4 reliably ignited a thermal plasma even at low flow rates atwhich ignition was difficult to perform by the conventional apparatus.Also, when the moisture-containing Ar gas was used, a thermal plasma wasignited with no problem in the present invention, whereas no ignitionwas possible in the conventional method.

[0195] In the second to fourth embodiments described above, freon gasand a gas containing water vapor are supplied into the annular gapformed between the outer and inner tubes 12 and 13 of the discharge tube11, thereby decomposing the freon. However, the present invention is notlimited to these embodiments. For example, it is also possible to supplythe organic halide into the inner tube 13 and supply water vapor intothe annular gap formed between the outer and inner tubes 12 and 13 togenerate a plasma, in the outer tube 12, which extends from the endportion of the inner tube 13 to the end portion of the outer tube 12,thereby decomposing the freon.

[0196] (Fifth Embodiment)

[0197]FIG. 7 is a sectional view showing an organic halide (e.g., freon)decomposing system including a microwave plasma generator having acylindrical microwave resonance cavity according to the fifth embodimentof the present invention. FIG. 8 is a sectional view of the microwaveplasma generator shown in FIG. 7.

[0198] Referring to FIG. 7, a horizontally extending rectangularwaveguide 41 has a microwave oscillator 42, which oscillates a microwavewith a frequency of 2.45 GHz, in its start end portion (left-hand end),and has a hole 43 in this end portion. This rectangular waveguide 41transmits a microwave from the start end portion toward a terminal endportion (right-hand end).

[0199] As shown in FIG. 8, a cylindrical microwave resonance cavity 44is so connected as to extend vertically by communicating with the hole43 of the rectangular waveguide 41. This cavity 44 includes an end plate45, a cylindrical outer conductor 46, a metal conductor 48, and acylindrical probe antenna 49. The end plate 45 is positioned on thebottom portion of the cavity 44, and the outer conductor 46 is fixed onthis end plate 45. The metal conductor 48 is fixed to the rectangularwaveguide 41 near the hole 43 and has an inverse conical portion 47extended into the outer conductor 46. A hole is formed through thecenter of this metal conductor 48. The probe antenna 49 is extended fromthe lower end of the inverse conical portion 47 of the metal conductor48 into the outer conductor 46 and is placed coaxially with the outerconductor 46. Reference numeral 44 a denotes a throttle plate which isformed on the upper surface of the cavity 44 and defines the hole 43 ofthe rectangular waveguide 41, i.e., of the connecting portion betweenthe rectangular waveguide 41 and the cavity 44. A reaction tube 50 isplaced below the coaxial microwave cavity 44.

[0200] A discharge tube 51 made of a dielectric material, e.g., quartz,runs through the end plate 45 from the hole in the metal conductor 48via the probe antenna 49 and is inserted into the reaction tube 50. Thatis, this discharge tube 51 is so placed as to be coaxial with thecentral axis of the cylindrical microwave resonance cavity 44 andextends through the rectangular waveguide 41 and the cavity 44.

[0201] Freon from a freon gas container 52, air from an air container53, and water vapor from water vapor generator 54 are supplied into theupper portion of the discharge tube 51 through a gas supply pipe 55.

[0202] The lower end of the reaction tube 50 is dipped into an aqueousalkali solution 57 in a vessel 56. An exhaust duct 58 is connected tothe upper portion of this vessel 56.

[0203] The function of the aforementioned microwave plasma generator anda method of decomposing an organic halide, e.g., freon, by using theorganic halide decomposing system including this generator will bedescribed below.

[0204] The microwave oscillator 42 is operated to generate a microwave.This microwave is transmitted by the rectangular waveguide 41 and thentransmitted to the cylindrical microwave resonance cavity 44 through themetal conductor 48 and the probe antenna 49. As a consequence, in thecavity 44 a large axial-direction electric field of TM₀₁₀ mode formsbetween the probe antenna 49 and the end plate 45. Additionally, thiselectric field in the cavity 44 is stable because the metal conductor 48and the probe antenna 49 couple the electric field mode in therectangular waveguide 41 and the electric field mode in the cylindricalmicrowave resonance cavity 44. Reference numeral 59 in FIG. 8 denotes anelectric field vector when the electric field of TM₀₁₀ mode is formed.

[0205] With the microwave thus transmitted into the cylindricalmicrowave resonance cavity 44, a gas containing an organic halide (e.g.,freon gas) is supplied into the discharge tube 51 through the gas supplypipe 55 and irradiated with the microwave. Consequently, a thermalplasma 60 having high electron energy and a temperature of 2,000 to6,000 K is generated in the discharge tube 51. This thermal plasma 60extends from the discharge tube 51 into the reaction tube 50 at thelower end of the discharge tube 51. This decomposes the freon gas intothe state in which it readily dissociates into chlorine, fluorine, andhydrogen atoms.

[0206] In this state, a large amount of energy absorption or the likeoccurs in the dissociation of the organic halide, and the loadfluctuation increases. However, an electric field of TM₀₁₀ mode havinglarge field strength forms, and the electric field mode in therectangular waveguide 41 is coupled with the electric field mode in thecylindrical microwave resonance cavity 44. Therefore, the organic halidecan be stably decomposed against the load fluctuation.

[0207] The decomposed gas from the reaction tube 50 is made harmless bypassing through the aqueous alkali solution (e.g., calcium hydroxide) 57contained in the vessel 56. The gas containing carbonic acid gas and thelike is exhausted from the exhaust duct 58.

EXAMPLE 5

[0208] In this Example 5, the organic halide decomposing systemincluding the microwave plasma generator according to the fifthembodiment described above was used to decompose freon R12 and freon134a under the conditions explained below.

[0209] That is, the inside diameter and length of the coaxial microwaveresonance cavity 44 for generating the thermal plasma 60 shown in FIGS.7 and 8 were set to 90 mm and 35 mm, respectively, and the gap lengthbetween the probe antenna 49 and the end plate 45 of the cavity 44 wasset to 15 mm. A quartz discharge tube 51 having an outer diameter of 1.2mm and an inner diameter of 11 mm was placed in the cavity 44 throughthe metal conductor 48, the probe antenna 49, and the end plate 45.

[0210] To a portion of the discharge tube 51 positioned inside thecavity 44, freon (R12) was supplied at the atmospheric pressure and aflow rate of 10 L/min from the freon container 52 through the gas supplypipe 55. At the same time, a 2.45-GHz microwave was introduced from themicrowave oscillator 42 into the coaxial microwave resonance cavity 44via the metal conductor 48 mounted on the rectangular waveguide 41 andthe inner conductor 49, thereby generating discharge by anaxial-direction electric field of TM₀₁₀ mode formed in this cavity 44.

[0211] This discharge was well stable even at the atmospheric pressurecompared to a conventional method. Also, the electric field vector 59obtained analytically maintained high electric field at the center ofthe coaxial microwave resonance cavity 44.

[0212] The freon gas was decomposed in the reaction tube 50 by thethermal plasma 60 discharged and was made harmless by passing throughthe aqueous alkali solution 57 (calcium hydroxide) in the vessel 56. Theremaining gas containing carbonic acid gas and the like was exhaustedfrom the exhaust duct 58.

[0213] The decomposition ratio of freon was measured by sampling aportion of the gas from the reaction tube 50 and calculating thepresence/absence of a plasma from gas chromatographic analysis of thefreon concentration. Table 6 below shows the experimental results ofdecomposition ratio measurements obtained when the freon supply amountwas 1 kg/h and the microwave power was used as a parameter.

[0214] The same test was conducted on freon 134a into which air wasmixed from the air container 53. The results are also shown in Table 6.TABLE 6 Freon supply Freon R12 Freon 134a amount Microwave decompositiondecomposition (kg/h) power (W) ratio (%) ratio (%) 1  700 90 85 1 100099.99 99 1 1200 99.99 99.99

[0215] As is apparent from Table 6, in Example 5 freon 134a wassimilarly decomposable.

[0216] Also, it was experimentally confirmed that even when argon, air,or the like was mixed as an additional gas, a substantially equaldecomposition ratio was obtained by controlling the microwave power.

[0217] (Sixth Embodiment)

[0218]FIG. 9 is a sectional view showing another form of the microwaveplasma generator having the coaxial microwave resonance cavityincorporated into the organic halide decomposing system according to thefifth embodiment described above. The same reference numerals as inFIGS. 7 and 8 denote the same parts in FIG. 9, and a detaileddescription thereof will be omitted.

[0219] As shown in FIG. 9, this microwave plasma generator has astructure in which a cylindrical slidable probe antenna 61 serving as atuner for field strength adjustment is interposed between a metalconductor 48 and this discharge tube 51 to be slidable in the axialdirection of a discharge tube 51. The metal conductor 48 is fixed to arectangular waveguide 41 near its hole 43 and surrounds the upperportion of the discharge tube 51. This metal conductor 48 exists in therectangular waveguide 41 but does not extend into a coaxial microwaveresonance cavity 44. The slidable probe antenna 61 transmits a microwavefrom the metal conductor 48 by slidably contacting the metal conductor48. This slidable probe antenna 61 extends into the cavity 44 throughthe hole 43.

[0220] In the microwave plasma generator with the above structure, theslidable probe antenna 61 is slid to adjust its length in the coaxialmicrowave resonance cavity 44. This allows the field strength to beadjusted in accordance with the load fluctuation of a thermal plasma 60generated in the discharge tube 51. Consequently, the operating powerrange can be widened with respect to the load fluctuation associatedwith changes in the plasma conditions. Hence, an organic halide can bedecomposed more effectively. Also, discharge can be stabilized even whenthe addition amount of a gas containing an organic halide and watervapor is changed.

[0221] A gas containing an organic halide is supplied into the dischargetube 51 of an organic halide decomposing system including the microwaveplasma generator shown in FIG. 9 and irradiated with a microwave by theplasma generator, thereby generating a thermal plasma. As a consequence,the organic halide can be easily decomposed as in the fifth embodimentdescribed above.

EXAMPLE 6

[0222] In this Example 6, the organic halide decomposing systemincluding the microwave plasma generator according to the sixthembodiment described above was used to decompose freon R12 under theconditions explained below.

[0223] That is, the field strength was adjusted with respect to the loadfluctuation of a plasma or the like generated in the discharge tube 51by controlling the length of the metal conductor inserted into thecoaxial microwave resonance cavity 44 from the hole 43 of therectangular waveguide 41 shown in FIG. 9, i.e., the length of theslidable probe antenna 61.

[0224] The freon decomposition ratio with respect to the length of theprobe antenna 61 was obtained in the same manner as in Example 5. Table7 below shows the experimental results of decomposition ratiomeasurements when the freon supply amount and the water vapor supplyamount were 1 kg/h and the microwave power was used as a parameter.TABLE 7 Water Freon vapor Probe supply supply length amount amountMicrowave Decomposition (mm) (kg/h) (kg/h) power (W) ratio (%)  5 1 11000 96 10 1 1 1000 99.99 15 1 1 1000 99

[0225] As shown in Table 7, freon could be decomposed more efficientlyby interposing, between the metal conductor 48 and the discharge tube51, the cylindrical slidable probe antenna 61, which serves as a tunerfor field strength adjustment, to be slidable in the axial direction ofthe discharge tube 51.

[0226] Also, even when argon, air, or the like was mixed as anadditional gas, a substantially equal decomposition ratio was obtainedby controlling the microwave power.

[0227] (Seventh Embodiment)

[0228]FIG. 10 is a sectional view showing still another form of themicrowave plasma generator having the coaxial microwave resonance cavityincorporated into the organic halide decomposing system according to thefifth embodiment described above. The same reference numerals as inFIGS. 7 and 8 denote the same parts in FIG. 10, and a detaileddescription thereof will be omitted.

[0229] As shown in FIG. 10, this microwave plasma generator has astructure in which an annular metal conductor 62 is slidably interposed,so as to extend from the exterior to the interior of a coaxial microwaveresonance cavity 44, between an end plate 45 of this cavity 44 and adischarge tube 51 extending through the end plate 45.

[0230] In the microwave plasma generator with the above structure, theenhancement amount of the strength of an electric field formed in thecavity 44 can be adjusted by slidably moving the annular metal conductor62 along the axial direction.

[0231] A gas containing an organic halide is supplied into the dischargetube 51 of an organic halide decomposing system including the microwaveplasma generator shown in FIG. 10 and irradiated with a microwave by theplasma generator, thereby generating a thermal plasma. As a consequence,the organic halide can be easily decomposed as in the fifth embodimentdescribed above.

EXAMPLE 7

[0232] In this Example 7, the organic halide decomposing systemincluding the microwave plasma generator according to the seventhembodiment described above was used to decompose freon R12 under theconditions explained below.

[0233] That is, freon R12 was decomposed following the same procedure asin Example 5 except that the field strength on the central axis of thecoaxial microwave resonance cavity 44 shown in FIG. 10 was enhanced bythe annular metal conductor 62 slidably interposed between the end plate45 of the cavity 44 and the discharge tube 51 extending through the endplate 45, and that water was sprayed upon the generated thermal plasma.

[0234] The freon decomposition ratio was obtained in the same manner asin Example 5. Table 8 below shows the experimental results ofdecomposition ratio measurements when the freon supply amount and thesprayed water supply amount were 1 kg/h and the insertion length of theannular metal conductor 62 was used as a parameter. TABLE 8 Freon WaterInsertion supply supply length amount amount Microwave Decomposition(mm) (kg/h) (kg/h) power (W) ratio (%) 0 1 1 500 90 5 1 1 450 98 10  1 1400 99.99

[0235] As shown in Table 8, freon R12 could be decomposed moreefficiently by slidably interposing the annular metal conductor 62between the end plate 45 of the coaxial microwave resonance cavity 44and the discharge tube 51 extending through this end plate 45.

[0236] Also, even when argon, air, or the like was mixed as anadditional gas, a substantially equal decomposition ratio was obtainedby controlling the microwave power.

[0237] (Eighth Embodiment)

[0238]FIG. 11 is a sectional view showing still another form of themicrowave plasma generator having the coaxial microwave resonance cavityincorporated into the organic halide decomposing system according to thefifth embodiment described above. The same reference numerals as inFIGS. 7 and 8 denote the same parts in FIG. 11, and a detaileddescription thereof will be omitted.

[0239] As shown in FIG. 11, this microwave plasma generator has astructure in which a tapered (conical) projection 63 which projectstoward a metal conductor 48 is formed on an end plate 45 of a coaxialmicrowave resonance cavity 44, through which a discharge tube 51 runs.

[0240] In the microwave plasma generator with the above structure, thetapered projection 63 is formed on the end plate 45 through which thedischarge tube 51 extends, so the field strength in the coaxialmicrowave resonance cavity 44 can be enhanced. Also, it is possible toprevent a thermal plasma 60 from contacting the discharge tube 51.

[0241] A gas containing an organic halide is supplied into the dischargetube 51 of an organic halide decomposing system including the microwaveplasma generator shown in FIG. 11 and irradiated with a microwave by theplasma generator, thereby generating a thermal plasma. As a consequence,the organic halide can be easily decomposed as in the fifth embodimentdescribed above.

EXAMPLE 8

[0242] In this Example 8, the organic halide decomposing systemincluding the microwave plasma generator according to the eighthembodiment described above was used to decompose freon R12 under theconditions explained below.

[0243] That is, freon R12 was decomposed following the same procedure asin Example 5 except that the tapered projection 63 projecting toward themetal conductor 48 was formed on the end plate 45 of the coaxialmicrowave resonance cavity 44, through which the discharge tube 51 ranas shown in FIG. 11, and that alkali water was sprayed into the reactiontube and allowed to flow along its wall surface.

[0244] The freon decomposition ratio was obtained in the same manner asin Example 5. Table 9 below shows the experimental results ofdecomposition ratio measurements based on the presence/absence of thetapered projection 63 on the end plate 45 when the freon supply amountwas 0.1 kg/h and the supply amount of the alkali water to be sprayed, inwhich calcium hydroxide was dissolved, was 1 kg/h. TABLE 9 Freon Watersupply supply Tapered amount amount Microwave Decomposition projection(kg/h) (kg/h) power (W) ratio (%) Not formed 1 1 1000 96 or more Formed1 1 1000 99

[0245] As can be seen from Table 9, freon R12 could be decomposed moreefficiently by forming the tapered projection 63 on the end plate 45through which the discharge tube 51 was extended.

[0246] Also, even when argon, air, or the like was mixed as anadditional gas, a substantially equal decomposition ratio was obtainedby controlling the microwave power.

[0247] In this Example 8, the method of allowing alkali water to flow onthe wall surface of the reaction tube 50 is described as an alkali watersupply method. However, alkali water can also be directly sprayed so asto surround the thermal plasma in the reaction tube 50.

[0248] (Ninth Embodiment)

[0249]FIG. 12 is a sectional view showing an organic halide (e.g.,freon) decomposing system including a microwave plasma generator havinga coaxial microwave cavity according to the ninth embodiment of thepresent invention.

[0250] Referring to FIG. 12, a horizontally extending rectangularwaveguide 71 has a microwave oscillator 72, which oscillates a microwavewith a frequency of 2.45 GHz, in its start end portion (left-hand end),and has a hole 73 in this end portion. This rectangular waveguide 71transmits a microwave from the start end portion toward a terminal endportion (right-hand end).

[0251] A coaxial microwave cavity 74 is so connected as to extendvertically by communicating with the hole 73 of the rectangularwaveguide 71. This cavity 74 includes an end plate 75, a cylindricalouter conductor 76, a metal conductor 77, and a cylindrical innerconductor 78. The end plate 75 is positioned on the bottom portion ofthe cavity 74, and the outer conductor 76 is integrated with this endplate 75. The metal conductor 77 is fixed to the rectangular waveguide71 near the hole 73 and extended into the outer conductor 76. A hole isformed through the center of this metal conductor 77. The innerconductor 78 runs from the lower end of this metal conductor 77 into theouter conductor 76 and is placed coaxially with the outer conductor 76.A reaction tube 79 is placed below the coaxial microwave cavity 74.

[0252] A discharge tube 80 made of a dielectric material, e.g., quartz,runs through the end plate 75 from the hole in the metal conductor 77via the inner conductor 78 and is inserted into the reaction tube 79.That is, this discharge tube 80 is so placed as to be coaxial with thecentral axis of the coaxial microwave cavity 74 and extends through therectangular waveguide 71 and the cavity 74.

[0253] Freon from a freon gas container 81 and air from an air container82 are supplied into the upper end portion of the discharge tube 80through a gas supply pipe 83.

[0254] The lower end of the reaction tube 79 is dipped into an aqueousalkali solution 85 in a vessel 84. An exhaust duct 86 is connected tothe upper portion of this vessel 84.

[0255] A method of decomposing an organic halide, e.g., freon, by usingthe organic halide decomposing system including the aforementionedmicrowave plasma generator will be described below.

[0256] The microwave oscillator 72 is operated to generate a microwave.This microwave is transmitted by the rectangular waveguide 71 and thentransmitted to the coaxial microwave cavity 74 through the metalconductor 77 and the inner conductor 78, As a consequence, in the cavity74 an axial-direction electric field forms between the inner conductor78 and the end plate 75.

[0257] With the microwave thus transmitted into the coaxial microwavecavity 74, a gas containing freon gas is supplied into the dischargetube 80 through the gas supply pipe 83 and irradiated with the microwavefrom the microwave plasma generator. Consequently, a thermal plasma 87having high electron energy and a temperature of 2,000 to 6,000 K isgenerated in the discharge tube 80. This thermal plasma 87 extends fromthe discharge tube 80 into the reaction tube 79 at the lower end of thedischarge tube 80. As a result, the freon gas is decomposed.

[0258] The decomposed gas from the reaction tube 79 is made harmless bypassing through the aqueous alkali solution (e.g., calcium hydroxide) 85contained in the vessel 84. The gas containing carbonic acid gas and thelike is exhausted from the exhaust duct 86.

EXAMPLE 9

[0259] In this Example 9, the organic halide decomposing systemincluding the microwave plasma generator according to the ninthembodiment described above was used to decompose freon 134a under theconditions explained below.

[0260] That is, the inside diameter and length of the coaxial microwavecavity 74 for generating a thermal plasma shown in FIG. 12 were set to40 mm and 50 mm, respectively, and the gap length between the innerconductor 78 and the end plate 75 of the cavity 74 was set to 10 mm.

[0261] Inside the cavity 74, a quartz discharge tube 80 having anoutside diameter of 12 mm and an inside diameter of 11 mm extendedthrough the metal conductor 77, the inner conductor 78, and the endplate 75.

[0262] To the discharge tube 80 in the cavity 74, freon 134a wassupplied at the atmospheric pressure and a flow rate of 10 L/min fromthe freon container 81 through the gas supply pipe 83. A 2.45-GHzmicrowave was introduced from the microwave oscillator 72 into thecoaxial microwave cavity 74 via the metal conductor 77 mounted on therectangular waveguide 71 and the inner conductor 78, thereby dischargingby an axial-direction electric field formed between the inner conductor78 and the end plate 75.

[0263] The decomposition ratio of freon was measured by sampling aportion of the gas from the reaction tube 79 and calculating thepresence/absence of a plasma from gas chromatographic analysis of thefreon concentration. Table 10 below shows the decomposition ratiomeasurement results obtained when the freon supply amount was 1 kg/h andthe microwave power was changed. TABLE 10 Freon supply MicrowaveDecomposition amount (kg/h) power (W) ratio (%) 0.1 400 80 0.1 300 750.1 200 50

[0264] As is apparent from Table 10, freon could be efficientlydecomposed by the method of Example 9.

[0265] Also, even when argon, air, or the like was mixed as anadditional gas, a substantially equal decomposition ratio was obtainedby controlling the microwave power.

[0266] (10th Embodiment)

[0267]FIG. 13 is a sectional view showing an organic halide (e.g.,freon) decomposing system including a microwave plasma generator havinga coaxial microwave cavity according to the 10th embodiment of thepresent invention. The same reference numerals as in FIG. 12 denote thesame parts in FIG. 13, and a detailed description thereof will beomitted.

[0268] This decomposing system has a structure in which a water supplysystem 89 is connected to a gas supply pipe 83 via a branch pipe 88, anda ribbon heater 90 as a heating means for heating water flowing in thebranch pipe 88 to generate water vapor is placed around the branch pipe88.

[0269] A method of decomposing an organic halide, e.g., freon, by usingthe organic halide decomposing system including aforementioned microwaveplasma generator will be described below.

[0270] A microwave oscillator 72 is operated to generate a microwave.This microwave is transmitted by a rectangular waveguide 71 and thentransmitted to a coaxial microwave cavity 74 through a metal conductor77 and an inner conductor 78. As a consequence, in the cavity 74 anaxial-direction electric field forms between the inner conductor 77 andan end plate 75.

[0271] With the microwave thus transmitted into the coaxial microwavecavity 74, water is supplied from the water supply system 89 to thebranch pipe 88. At the same time, this water flowing in the branch pipe88 was heated by the ribbon heater 90 to supply water vapor to the gassupply pipe 83, and freon is supplied from a freon container 81 to thegas supply pipe 83, thereby supplying a gas containing the freon and thewater vapor to a discharge tube 80. Since this gas is irradiated withthe microwave from the microwave plasma generator, a thermal plasma 87having high electron energy and a temperature of 2,000 to 6,000 K isgenerated in the discharge tube 80. This thermal plasma 87 extends fromthe discharge tube 80 into a reaction tube 79 at the lower end of thedischarge tube 80. This makes the freon gas readily dissociable intochlorine, fluorine, and hydrogen atoms, so the freon gas reacts with thewater vapor. For example, freon R12 is readily decomposed by reaction asindicated by formula 1 presented earlier.

[0272] The decomposed gas from the reaction tube 79 is made harmless bypassing through an aqueous alkali solution (e.g., calcium hydroxide) 85contained in a vessel 84. The gas containing carbonic acid gas and thelike is exhausted from an exhaust duct 86.

EXAMPLE 10

[0273] In this Example 10, the organic halide decomposing systemincluding the microwave plasma generator according to the 10thembodiment described above was used to decompose freon 134a under theconditions explained below.

[0274] That is, freon 134a was decomposed following the same proceduresas in Example 9 except that, as shown in FIG. 13, water was suppliedfrom the water supply system 89 to the branch pipe 88, andsimultaneously this water flowing in the branch pipe 88 was heated bythe ribbon heater 90 to supply water vapor to the gas supply pipe 83 andfreon 134a was supplied from the freon container 81 to the gas supplypipe 83 and to the discharge tube 80.

[0275] The freon decomposition ratio was measured following the sameprocedure as in Example 9. Table 11 below shows the experimental resultsof decomposition ratio measurements obtained when the freon supplyamount and the water vapor supply amount were 0.1 kg/h and the microwavepower was used as a parameter. TABLE 11 Water Freon vapor supply supplyMicrowave amount amount output Decomposition (kg/h) (kg/h) (W) ratio (%)0.1 0.1 400 99 or more 0.1 0.1 300 99 0.1 0.1 200 95

[0276] As is apparent from Table 11, the method of Example 10 by whichwater vapor was supplied together with freon to the discharge tube 80could further improve the freon decomposition efficiency, compared toExample 9.

[0277] Also, even when argon, air, or the like was mixed as anadditional gas, a substantially equal decomposition ratio was obtainedby controlling the microwave power.

[0278] In this Example 10, the method of thermally vaporizing waterflowing in the branch pipe by using the ribbon heater is explained as awater vapor supply method. However, similar effects can be obtained bybubbling air in a vessel heated by a heater or the like and supplyingwater vapor, corresponding to the saturation vapor pressure, togetherwith freon into the discharge tube.

[0279] (11th Embodiment)

[0280]FIG. 14 is a sectional view showing an organic halide (e.g.,freon) decomposing system including a microwave plasma generator havinga coaxial microwave cavity according to the 11th embodiment of thepresent invention. The same reference numerals as in FIG. 12 denote thesame parts in FIG. 14, and a detailed description thereof will beomitted.

[0281] As shown in FIG. 14, this decomposing system has a structure inwhich two, for example, water spray nozzles 91 are inserted into thecircumferential wall of a reaction tube 79 such that the sprayed streamspoint to a thermal plasma generation region at the lower end of adischarge tube 80.

[0282] A method of decomposing an organic halide, e.g., freon, by usingthe organic halide decomposing system including aforementioned microwaveplasma generator will be described below.

[0283] A microwave oscillator 72 is operated to generate a microwave.This microwave is transmitted by a rectangular waveguide 71 and thentransmitted to a coaxial microwave cavity 74 through a metal conductor77 and an inner conductor 78. As a consequence, in the cavity 74 anaxial-direction electric field forms between the inner conductor 78 andan end plate 75.

[0284] With the microwave thus transmitted into the coaxial microwavecavity 74, a gas containing freon is supplied from a freon container 81into the discharge tube 80 through a gas supply pipe 83. Since this gasis irradiated with the microwave from the microwave plasma generator, athermal plasma 87 having high electron energy and a temperature of 2,000to 6,000 K is generated to extend from the discharge tube 80 into thereaction tube 79 at the lower end of the discharge tube 80. This makesthe freon gas readily dissociable into chlorine, fluorine, and hydrogenatoms. In this state, water 92 is sprayed upon the thermal plasma 87from the two water spray nozzles 91 and converted into water vapor.Consequently, the freon in the abovementioned easily dissociable statereacts with the water vapor. For example, freon R12 is readilydecomposed by reacting as indicated by formula 1 presented earlier.

[0285] The decomposed gas from the reaction tube 79 is made harmless bypassing through an aqueous alkali solution (e.g., calcium hydroxide) 85contained in a vessel 84. The gas containing carbonic acid gas and thelike is exhausted from an exhaust duct 86.k

EXAMPLE 11

[0286] In this Example 11, the organic halide decomposing systemincluding the microwave plasma generator according to the 11thembodiment described above was used to decompose freon 134a under theconditions explained below.

[0287] That is, freon 134a was decomposed following the same proceduresas in Example 9 except that, as shown in FIG. 14, the water 92 wassprayed upon the thermal plasma 87 from the two water spray nozzles 91.

[0288] The freon decomposition ratio was measured following the sameprocedure as in Example 9. Table 12 below shows the experimental resultsof decomposition ratio measurements obtained when the freon supplyamount and the sprayed water supply amount were 0.1 kg/h and themicrowave power was used as a parameter. TABLE 12 Water Freon vaporsupply supply Microwave amount amount output Decomposition (kg/h) (kg/h)(W) ratio (%) 0.1 0.1 500 99 or more 0.1 0.1 450 98 0.1 0.1 400 90

[0289] As shown in Table 12, the method of Example 11 by which watervapor was supplied together with freon to the discharge tube 80 couldfurther improve the freon decomposition efficiency, compared to Example9.

[0290] Also, even when argon, air, or the like was mixed as anadditional gas, a substantially equal decomposition ratio was obtainedby controlling the microwave power.

[0291] In this Example 11, the method of spraying water from the twowater spray nozzles opposing each other is described as a water supplymethod. However, the decomposition ratio can further improve when waterspray nozzles are installed such that water is sprayed all over theplasma in the reaction tube.

[0292] (12th Embodiment)

[0293]FIG. 15 is a sectional view showing an organic halide (e.g.,freon) decomposing system including a microwave plasma generator havinga coaxial microwave cavity according to the 12th embodiment of thepresent invention. The same reference numerals as in FIG. 12 denote thesame parts in FIG. 15, and a detailed description thereof will beomitted.

[0294] As shown in FIG. 15, this decomposing system has a structure inwhich two, for example, alkali water supply pipes 93 are connected tothe circumferential wall of a reaction tube 79 such that their endportions oppose a thermal plasma generation region at the lower end of adischarge tube 80.

[0295] A method of decomposing an organic halide, e.g., freon, by usingthe organic halide decomposing system including aforementioned microwaveplasma generator will be described below.

[0296] A microwave oscillator 72 is operated to generate a microwave.This microwave is transmitted by a rectangular waveguide 71 and thentransmitted to a coaxial microwave cavity 74 through a metal conductor77 and an inner conductor 78. As a consequence, in the cavity 74 anaxial-direction electric field forms between the inner conductor 77 andan end plate 75.

[0297] With the microwave thus transmitted into the coaxial microwavecavity 74, a gas containing freon is supplied from a freon container 81into the discharge tube 80 through a gas supply pipe 83. Since this gasis irradiated with the microwave from the microwave plasma generator, athermal plasma 87 having high electron energy and a temperature of 2,000to 6,000 K is generated to extend from the discharge tube 80 into thereaction tube 79 at the lower end of the discharge tube 80. This makesthe freon gas readily dissociable into chlorine, fluorine, and hydrogenatoms. In this state, alkali water 94 is supplied from the alkali watersupply pipes 93 along the inner surface of the circumferential wall ofthe reaction tube 79, thereby generating water vapor by the heat of thethermal plasma 87. Consequently, the freon in the abovementioned easilydissociable state reacts with the water vapor. For example, freon R12 isreadily decomposed by reaction indicated by formula 1 presented earlier.At the same time, the decomposed gas reacts with alkali water (e.g., anaqueous calcium hydroxide solution) and is made harmless in the form ofa halide salt.

[0298] The decomposed gas which is not rendered harmless from thereaction tube 79 is made harmless by passing through an aqueous alkalisolution (e.g., calcium hydroxide) 85 contained in a vessel 84. A gascontaining carbonic acid gas and the like is exhausted from an exhaustduct 86.

EXAMPLE 12

[0299] In this Example 12, the organic halide decomposing systemincluding the microwave plasma generator according to the 12thembodiment described above was used to decompose freon 134a (CH₂CF₄)under the conditions explained below.

[0300] That is, freon 134a was decomposed following the same proceduresas in Example 9 except that, as shown in FIG. 15, the alkali water(aqueous calcium hydroxide solution) 94 was supplied from the two alkaliwater supply pipes 93 along the inner surface of the circumferentialwall of the reaction tube 79.

[0301] The freon decomposition ratio was measured following the sameprocedure as in Example 9. Table 13 below shows the experimental resultsof decomposition ratio measurements obtained when the freon supplyamount and the supply amount of alkali water to be sprayed, in whichcalcium hydroxide was dissolved, were 0.1 kg/h and the microwave powerwas used as a parameter. TABLE 13 Alkali Freon water supply supplyMicrowave amount amount output Decomposition (kg/h) (kg/h) (W) ratio (%)0.1 0.1 450 99 or more 0.1 0.1 400 98 0.1 0.1 300 90

[0302] As shown in Table 13, the method of Example 12 by which alkaliwater was supplied along the inner surface of the circumferential wallof the reaction tube 79 could further improve the freon decompositionefficiency, compared to Example 9.

[0303] Also, even when argon, air, or the like was mixed as anadditional gas, a substantially equal decomposition ratio was obtainedby controlling the microwave power.

[0304] In this Example 12, the method of supplying alkali water alongthe inner wall surface of the reaction tube is described as an alkaliwater supply method. However, alkali water can also be directly sprayedto surround the plasma in the reaction tube.

[0305] (13th Embodiment)

[0306]FIG. 16 is a sectional view showing an organic halide (e.g.,freon) decomposing system including a microwave plasma generator havinga coaxial microwave cavity according to the 13th embodiment of thepresent invention. The same reference numerals as in FIG. 12 denote thesame parts in FIG. 16, and a detailed description thereof will beomitted.

[0307] As shown in FIG. 16, this decomposing system has a structure inwhich heaters 95 and 96 are installed around the circumferential wall ofa reaction tube 79 and below the bottom of a vessel 84, respectively.

[0308] Note that heaters need not be installed on both thecircumferential wall of the reaction tube 79 and the bottom of thevessel 84; one heater need only be installed on one of them. However, tosupply an enough amount of water vapor into the reaction tube 79,heaters are preferably installed on both members.

[0309] A method of decomposing an organic halide, e.g., freon, by usingthe organic halide decomposing system including aforementioned microwaveplasma generator will be described below.

[0310] A microwave oscillator 72 is operated to generate a microwave.This microwave is transmitted by a rectangular waveguide 71 and thentransmitted to a coaxial microwave cavity 74 through a metal conductor77 and an inner conductor 78. As a consequence, in the cavity 74 anaxial-direction electric field is formed between the inner conductor 77and an end plate 75.

[0311] With the microwave thus transmitted into the coaxial microwavecavity 74 as described above, a gas containing freon is supplied from afreon container 81 into a discharge tube 80 through a gas supply pipe83. Since this gas is irradiated with the microwave from the microwaveplasma generator, a thermal plasma 87 having high electron energy and atemperature of 2,000 to 6,000 K is generated to extend from thedischarge tube 80 into the reaction tube 79 at the lower end of thedischarge tube 80. This makes the freon gas readily dissociable intochlorine, fluorine, and hydrogen atoms. In this state, the heaters 95and 96 heat the reaction tube 79 and the vessel 84 to vaporize anaqueous alkali solution 85 contained in the vessel 84, thereby supplyingwater vapor to the thermal plasma 87. Consequently, the freon in theabovementioned easily dissociable state reacts with the water vapor. Forexample, freon R12 is readily decomposed by reaction indicated byformula 1 presented earlier.

[0312] The decomposed gas from the reaction tube 79 is made harmless bypassing through the aqueous alkali solution (e.g., calcium hydroxide) 85contained in the vessel 84. A gas containing carbonic acid gas and thelike is exhausted from an exhaust duct 86.

EXAMPLE 13

[0313] In this Example 13, the organic halide decomposing systemincluding the microwave plasma generator according to the 13thembodiment described above was used to decompose freon 134a under theconditions explained below.

[0314] That is, freon 134a was decomposed following the same proceduresas in Example 9 except that, as shown in FIG. 16, the reaction tube 79and the vessel 84 were heated to a temperature of 85° C. by the heaters95 and 96 to vaporize the aqueous alkali water 85 contained in thevessel 84, thereby supplying water vapor to the thermal plasma 87generated in the discharge tube 80.

[0315] The freon decomposition ratio was measured following the sameprocedure as in Example 9. Table 14 below shows the experimental resultsof decomposition ratio measurements obtained when the freon supplyamount was 0.1 kg/h, alkali water was heated to 85° C., and themicrowave power was used as a parameter. TABLE 14 Alkali Freon watersupply supply Microwave amount amount output Decomposition (kg/h) (kg/h)(W) ratio (%) 0.1 85 600 99 or more 0.1 85 500 98 0.1 85 450 90

[0316] As can be seen from Table 14, the method of Example 13 by whichthe heaters 95 and 96 heated the reaction tube 79 and the vessel 84,respectively, to vaporize the aqueous alkali water 85 contained in thevessel 84 and thereby introduced water vapor into the reaction tube 79could further improve the freon decomposition efficiency, compared toExample 9.

[0317] Also, even when argon, air, or the like was mixed as anadditional gas, a substantially equal decomposition ratio was obtainedby controlling the microwave power.

[0318] A microwave plasma generator according to the present inventionhas an effect of stably generating a thermal plasma consisting of a gasmixture of an organic halide, such as freon, and water vapor. In adischarge tube having a double-tube structure, the gap between inner andouter tubes has neither a tapered portion nor a narrowed flow path. Thismaintains a swirling flow effect and can prevent melting of thedischarge tube and adhesion of solid matter. Also, since a thermalplasma can be stably generated, the consumption of argon can besuppressed.

[0319] Another microwave plasma generator according to the presentinvention can stably maintain continuous discharge for long timeperiods. Also, high microwave input power and abrupt fluctuations of theload can be easily controlled. Furthermore, a cylindrical microwaveresonance cavity can be adjusted in accordance with the load, sodischarge can be readily induced in accordance with the sample.

[0320] In an organic halide decomposing method according to the presentinvention, organic halides such as freon and trichloromethane in wastesor exhaust gases, which are conventionally difficult to decompose, canbe made harmless at a high decomposition ratio (99.99% or more).

[0321] Furthermore, in an organic halide decomposing system according tothe present invention, a cylindrical microwave resonance cavityefficiently and convergently supplies high microwave power to a gascontaining an organic halide. Therefore, a stable plasma can beefficiently generated, and the size and cost of the apparatus can bedecreased.

[0322] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A microwave plasma generator comprising: arectangular waveguide having a hole to transmit a microwave; a coaxialmicrowave cavity connected to said rectangular waveguide bycommunicating with the hole; and a discharge tube which is made of adielectric material, extends through the hole of said rectangularwaveguide, and extends through said cavity so as to be coaxial with acentral axis of said cavity, wherein said discharge tube has adouble-tube structure including outer and inner tubes, a sectional areaof an annular gap formed between said outer and inner tubes is constantover an entire length of said inner tube, and an end portion of saidinner tube has a gas injection opening having the same diameter as aninternal cavity of said inner tube.
 2. A generator according to claim 1,further comprising an ignition electrode installed in said inner tube togenerate microwave discharge.
 3. A generator according to claim 1,further comprising an ignition coil installed in said inner tube togenerate microwave discharge.
 4. A generator according to any one ofclaims 1 to 3, further comprising a gas supply pipe installed in the gapbetween said outer and inner tubes along the direction of tangent ofsaid outer tube.
 5. A microwave plasma generator comprising: arectangular waveguide having a hole to transmit a microwave; acylindrical microwave resonance cavity which is connected to saidrectangular waveguide by communicating with the hole and is placed suchthat a central axis of said resonance cavity aligns with the directionof electric field in said rectangular waveguide; and a discharge tubewhich is made of a dielectric material, extends through the hole of saidrectangular waveguide, and extends through said resonance cavity so asto be coaxial with the central axis of said cavity, wherein saiddischarge tube has a double-tube structure including outer and innertubes, a sectional area of an annular gap formed between said outer andinner tubes is constant over an entire length of said inner tube, and anend portion of said inner tube has a gas injection opening having thesame diameter as an internal cavity of said inner tube.
 6. A generatoraccording to claim 5, further comprising an ignition electrode installedin said inner tube to generate microwave discharge.
 7. A generatoraccording to claim 5, further comprising an ignition coil installed insaid inner tube to generate microwave discharge.
 8. A generatoraccording to any one of claims 5 to 7, further comprising a gas supplypipe installed in the gap between said outer and inner tubes along thedirection of tangent of said outer tube.
 9. A method of decomposing anorganic halide, which decomposes an organic halide in a plasma by usinga microwave plasma generator comprising a rectangular waveguide having ahole to transmit a microwave, a coaxial microwave cavity connected tosaid rectangular waveguide by communicating with the hole, and adischarge tube which is made of a dielectric material, extends throughthe hole of said rectangular waveguide, and extends through said cavityso as to be coaxial with a central axis of said cavity, wherein saiddischarge tube has a double-tube structure including outer and innertubes, a sectional area of an annular gap formed between said outer andinner tubes is constant over an entire length of said inner tube, and anend portion of said inner tube has a gas injection opening having thesame diameter as an internal cavity of said inner tube, wherein a gascontaining the organic halide, water vapor, and air is supplied to theannular gap formed between said outer and inner tubes to generate aplasma, in said outer tube, which extends from the vicinity of the endportion of said inner tube toward an end portion of said outer tube. 10.A method of decomposing an organic halide, which decomposes an organichalide in a plasma by using a microwave plasma generator comprising arectangular waveguide having a hole to transmit a microwave, a coaxialmicrowave cavity connected to said rectangular waveguide bycommunicating with the hole, and a discharge tube which is made of adielectric material, extends through the hole of said rectangularwaveguide, and extends through said cavity so as to be coaxial with acentral axis of said cavity, wherein said discharge tube has adouble-tube structure including outer and inner tubes, a sectional areaof an annular gap formed between said outer and inner tubes is constantover an entire length of said inner tube, and an end portion of saidinner tube has a gas injection opening having the same diameter as aninternal cavity of said inner tube, wherein the organic halide issupplied into said inner tube, and a gas containing water vapor and airis supplied to the annular gap formed between said outer and innertubes, thereby generating a plasma, in said outer tube, which extendsfrom the vicinity of the end portion of said inner tube toward an endportion of said outer tube.
 11. A method of decomposing an organichalide, which decomposes an organic halide in a plasma by using amicrowave plasma generator comprising a rectangular waveguide having ahole to transmit a microwave, a cylindrical microwave resonance cavityconnected to said rectangular waveguide by communicating with the holeand placed such that a central axis of said resonance cavity aligns withthe direction of electric field in said rectangular waveguide, and adischarge tube which is made of a dielectric material, extends throughthe hole of said rectangular waveguide, and extends through saidresonance cavity so as to be coaxial with the central axis of saidcavity, wherein said discharge tube has a double-tube structureincluding outer and inner tubes, a sectional area of an annular gapformed between said outer and inner tubes is constant over an entirelength of said inner tube, and an end portion of said inner tube has agas injection opening having the same diameter as an internal cavity ofsaid inner tube, a gas containing the organic halide, water vapor, andair is supplied to the annular gap formed between said outer and innertubes to generate a plasma, in said outer tube, which extends from thevicinity of the end portion of said inner tube toward an end portion ofsaid outer tube.
 12. A method of decomposing an organic halide, whichdecomposes an organic halide in a plasma by using a microwave plasmagenerator comprising a rectangular waveguide having a hole to transmit amicrowave, a cylindrical microwave resonance cavity connected to saidrectangular waveguide by communicating with the hole and placed suchthat a central axis of said resonance cavity aligns with the directionof electric field in said rectangular waveguide, and a discharge tubewhich is made of a dielectric material, extends through the hole of saidrectangular waveguide, and extends through said resonance cavity so asto be coaxial with the central axis of said cavity, wherein saiddischarge tube has a double-tube structure including outer and innertubes, a sectional area of an annular gap formed between said outer andinner tubes is constant over an entire length of said inner tube, and anend portion of said inner tube has a gas injection opening having thesame diameter as an internal cavity of said inner tube, the organichalide is supplied into said inner tube, and a gas containing watervapor and air is supplied to the annular gap formed between said outerand inner tubes, thereby generating a plasma, in said outer tube, whichextends from the vicinity of the end portion of said inner tube towardan end portion of said outer tube.
 13. A microwave plasma generatorcomprising: a rectangular waveguide having a hole to transmit amicrowave; a cylindrical microwave resonance cavity which is connectedto said rectangular waveguide by communicating with the hole and isplaced such that a central axis of said resonance cavity aligns with thedirection of electric field in said rectangular waveguide; a dischargetube which is made of a dielectric material, extends through the hole ofsaid rectangular waveguide, and extends through said resonance cavity soas to be coaxial with the central axis of said cavity; and a metalconductor which is connected, while being fitted in said discharge tube,to a portion of said discharge tube near the hole, and extends into saidresonance cavity through the hole.
 14. A generator according to claim13, wherein said cylindrical microwave resonance cavity internally formsan electric field of TM₀₁₀ mode.
 15. A microwave plasma generatorcomprising: a rectangular waveguide having a hole to transmit amicrowave; a cylindrical microwave resonance cavity which is connectedto said rectangular waveguide by communicating with the hole and isplaced such that a central axis of said resonance cavity aligns with thedirection of electric field in said rectangular waveguide; a dischargetube which is made of a dielectric material, extends through the hole ofsaid rectangular waveguide, and extends through said resonance cavity soas to be coaxial with the central axis of said cavity; a metal conductorwhich is connected, while being fitted in said discharge tube, to aportion of said discharge tube near the hole, and extends into saidresonance cavity through the hole; and a slidable probe antenna which isinterposed between said metal conductor and said discharge tube so as tobe slidable in an axial direction of said discharge tube, and extendsinto said resonance cavity through the hole in said rectangularwaveguide.
 16. A generator according to claim 15, wherein saidcylindrical microwave resonance cavity internally forms an electricfield of TM₀₁₀ mode.
 17. A microwave plasma generator comprising: arectangular waveguide having a hole to transmit a microwave; acylindrical microwave resonance cavity which is connected to saidrectangular waveguide by communicating with the hole, is placed suchthat a central axis of said resonance cavity aligns with the directionof electric field in said rectangular waveguide, and has an end plate ona bottom portion; a discharge tube which is made of a dielectricmaterial, extends through the hole of said rectangular waveguide, andextends through said resonance cavity so as to be coaxial with thecentral axis of said cavity; a metal conductor which is connected, whilebeing fitted in said discharge tube, to a portion of said discharge tubenear the hole, and extends into said resonance cavity through the hole;and an annular metal conductor which is interposed between said endplate of said resonance cavity and said discharge tube extending throughsaid end plate, and extends from the outside to the inside of saidcavity.
 18. A generator according to claim 17, wherein said cylindricalmicrowave resonance cavity internally forms an electric field of TM₀₁₀mode.
 19. A microwave plasma generator comprising: a rectangularwaveguide having a hole to transmit a microwave; a cylindrical microwaveresonance cavity which is connected to said rectangular waveguide bycommunicating with the hole, is placed such that a central axis of saidresonance cavity aligns with the direction of electric field in saidrectangular waveguide, and has an end plate on a bottom portion; adischarge tube made of a dielectric material, extends through the holeof said rectangular waveguide, and extends through said resonance cavityso as to be coaxial with the central axis of said cavity; and a metalconductor which is connected, while being fitted in said discharge tube,to a portion of said discharge tube near the hole, and extends into saidresonance cavity through the hole, wherein said end plate of saidresonance cavity has a tapered projection, which projects toward saidmetal conductor, in a portion through which said discharge tube extends.20. A generator according to claim 19, wherein said cylindricalmicrowave resonance cavity internally forms an electric field of TM₀₁₀mode.
 21. A method of decomposing an organic halide, wherein a thermalplasma is generated by irradiating a gas containing an organic halidewith a microwave, thereby decomposing the organic halide.
 22. A methodaccording to claim 21, wherein the gas is a gas mixture of the organichalide and water vapor.
 23. A method according to claim 21, whereinwater is sprayed into the thermal plasma of the gas containing theorganic halide.
 24. A method according to claim 21, alkali water issprayed into the thermal plasma of the gas containing the organichalide, thereby decomposing the organic halide and making a productharmless.
 25. A system for decomposing an organic halide, comprising: arectangular waveguide having a hole to transmit a microwave; a coaxialmicrowave cavity connected to said rectangular waveguide bycommunicating with the hole; a reaction tube placed below said cavity; adischarge tube made of a dielectric material, extending through the holeof said rectangular waveguide and extending into said reaction tubethrough said cavity so as to be coaxial with the central axis of saidcavity; a metal conductor which is connected, while being fitted in saiddischarge tube, to a portion of said discharge tube near the hole, andextends into said resonance cavity through the hole; a vessel into whicha lower end of said reaction tube is inserted and which contains anaqueous alkali solution; and gas supply means for supplying a gascontaining an organic halide to said discharge tube.
 26. A system fordecomposing an organic halide, comprising: a rectangular waveguidehaving a hole to transmit a microwave; a coaxial microwave cavityconnected to said rectangular waveguide by communicating with the hole;a reaction tube placed below said cavity; a discharge tube which is madeof a dielectric material, extends through the hole of said rectangularwaveguide, and extends into said reaction tube through said cavity so asto be coaxial with the central axis of said cavity; a metal conductorwhich is connected, while being fitted in said discharge tube, to aportion of said discharge tube near the hole, and extends into saidresonance cavity through the hole; a vessel into which a lower end ofsaid reaction tube is inserted and which contains an aqueous alkalisolution; gas supply means for supplying a gas containing an organichalide to said discharge tube through first piping; water supply meansconnected to said first piping through second piping; and heating means,provided for said second piping, for converting water flowing in saidsecond piping into water vapor.
 27. A system for decomposing an organichalide, comprising: a rectangular waveguide having a hole to transmit amicrowave; a coaxial microwave cavity connected to said rectangularwaveguide by communicating with the hole; a reaction tube placed belowsaid cavity; a discharge tube which is made of a dielectric material,extends through the hole of said rectangular waveguide, and extends intosaid reaction tube through said cavity so as to be coaxial with thecentral axis of said cavity; a metal conductor which is connected, whilebeing fitted in said discharge tube, to a portion of said discharge tubenear the hole, and extends into said resonance cavity through the hole;a vessel into which a lower end of said reaction tube is inserted andwhich contains an aqueous alkali solution; gas supply means forsupplying a gas containing an organic halide to said discharge tube; andwater vapor spraying means, formed in a circumferential wall of saidreaction tube, for spraying water vapor into a thermal plasma generationregion near a lower end portion of said discharge tube.
 28. A system fordecomposing an organic halide, comprising: a rectangular waveguidehaving a hole to transmit a microwave; a coaxial microwave cavityconnected to said rectangular waveguide by communicating with the hole;a reaction tube placed below said cavity; a discharge tube which is madeof a dielectric material, extends through the hole of said rectangularwaveguide, and extends into said reaction tube through said cavity so asto be coaxial with the central axis of said cavity; a metal conductorwhich is connected, while being fitted in said discharge tube, to aportion of said discharge tube near the hole, and extends into saidresonance cavity through the hole; a vessel into which a lower end ofsaid reaction tube is inserted and which contains an aqueous alkalisolution; gas supply means for supplying a gas containing an organichalide to said discharge tube; and alkali water spraying means, formedin a side wall of said reaction tube, for spraying alkali water into athermal plasma generation region near a lower end portion of saiddischarge tube.
 29. A system for decomposing an organic halide,comprising: a rectangular waveguide having a hole to transmit amicrowave; a coaxial microwave cavity connected to said rectangularwaveguide by communicating with the hole; a reaction tube placed belowsaid cavity; a discharge tube which is made of a dielectric material,extends through the hole of said rectangular waveguide, and extends intosaid reaction tube through said cavity so as to be coaxial with thecentral axis of said cavity; a metal conductor which is connected, whilebeing fitted in said discharge tube, to a portion of said discharge tubenear the hole, and extends into said resonance cavity through the hole;a vessel into which a lower end of said reaction tube is inserted andwhich contains an aqueous alkali solution; gas supply means forsupplying a gas containing an organic halide to said discharge tube; andheating means, installed in at least one of a position around saidreaction tube and a position below said vessel, for vaporizing theaqueous alkali solution in said vessel and introducing water vapor intoa thermal plasma generation region near a lower end portion of saiddischarge tube.